The present invention relates to highway guardrail systems having a guardrail mounted on posts, and more particularly, to guardrail end treatments designed to meet applicable federal and state safety standards including but not limited to crash worthiness requirements.
Along most highways there are hazards which present substantial danger to drivers and passengers of vehicles if the vehicles leave the highway. To prevent accidents from a vehicle leaving a highway, guardrail systems are often provided along the side of the highway. Experience has shown that guardrails should be installed such that the end of a guardrail facing oncoming traffic does not present another hazard more dangerous than the original hazard requiring installation of the associated guardrail systems. Early guardrail systems often had no protection at the end facing oncoming traffic. Sometimes impacting vehicles became impaled on the end of the guardrail causing extensive damage to the vehicle and severe injury to the driver and/or passengers. In some reported cases, the guardrail penetrated directly into the passenger""s compartment of the vehicle fatally injuring the driver and passengers.
Various highway guardrail systems and guardrail end treatments have been developed to minimize the consequences resulting from a head-on impact between a vehicle and the extreme end of the associated guardrail. One example of such end treatments includes tapering the ends of the associated guardrail into the ground to eliminate potential impact with the extreme end of the guardrail. Other types of end treatments include breakaway cable terminals (BCT), vehicle attenuating terminals (VAT), the SENTRE end treatment, and breakaway end terminals (BET).
It is desirable for an end terminal assembly installed at one end of a guardrail facing oncoming traffic to attenuate any head-on impact with the end of the guardrail and to provide an effective anchor to redirect a vehicle back onto the associated roadway after a rail face impact with the guardrail downstream from the end terminal assembly. Examples of such end treatments are shown in U.S. Pat. No. 4,928,928 entitled Guardrail Extruder Terminal, and U.S. Pat. No. 5,078,366 entitled Guardrail Extruder Terminal.
A SENTRE end treatment often includes a series of breakaway steel guardrail support posts and frangible plastic containers filled with sandbags. An impacting vehicle is decelerated as the guardrail support posts release or shear and the plastic containers and sandbags are compacted. A cable is often included to guide an impacting vehicle away from the associated guardrail.
A head-on collision with a guardrail support post located at the end of a guardrail system may result in vaulting the impacting vehicle. Therefore, guardrail end treatments often include one or more breakaway support posts which will yield or shear upon impact by a vehicle. Examples of previously available breakaway posts are shown in U.S. Pat. No. 4,784,515 entitled Collapsible Highway Barrier and U.S. Pat. No. 4,607,824 entitled Guardrail End Terminal. Posts such as shown in the ""515 and the ""824 Patents include a slip base with a top plate and a bottom plate which are designed to not yield upon lateral impact. When sufficient axial impact force is applied to the upper portion of the associated post, the top plate and the bottom plate will slide relative to each other. If a vehicle contacts the upper part of the post, the associated impact forces tend to produce a bending moment which may reduce or eliminate any slipping of the top plate relative to the bottom plate. Also, improper installation of the top plate relative to the bottom plate, such as over tightening of the associated mechanical fasteners, may prevent proper functioning of the slip base. A breakaway support post is also shown in U.S. Pat. No. 5,503,495 entitled Thrie-Beam Terminal with Breakaway Post Cable Release.
Wooden breakaway support posts are frequently used to releasably anchor guardrail end treatments and portions of the associated guardrail. Such wooden breakaway support posts, when properly installed, generally perform satisfactorily to minimize damage to an impacting vehicle during either a rail face impact or a head-on impact. However, impact of a vehicle with a wooden breakaway support post may often result in substantial damage to the adjacent soil. Removing portions of a broken wooden post from the soil is often both time consuming and further damages the soil. Therefore, wooden breakaway support posts are often installed in hollow metal tubes, sometimes referred to as foundation sleeves, and/or concrete foundations. For some applications, one or more soil plates may be attached to each metal sleeve to further improve the breakaway characteristics of the associated wooden post. Such metal sleeves and/or concrete foundations are relatively expensive and time consuming to install.
Light poles, sign posts or similar items are often installed next to a roadway with a breakable or releasable connection. For some applications, a cement foundation may be provided adjacent to the roadway with three or more bolts projecting from the foundation around the circumference of the pole. Various types of frangible or breakable connections may be formed between the bolts and portions of the light pole or sign post.
In accordance with teachings of the present invention, various shortcomings of previous guardrail support posts associated with highway guardrail end treatments have been addressed. The present invention provides a breakaway support post which will buckle or yield during head-on impact by a vehicle at or near the extreme end of an associated guardrail to minimize damage to the vehicle and provide sufficient strength to direct a vehicle back onto an associated roadway during a rail face impact with the guardrail downstream from the guardrail end treatment. The use of breakaway support posts incorporating teachings of the present invention substantially reduces the time and cost associated with initial installation of a guardrail end treatment and repair of the guardrail end treatment following impact by a motor vehicle.
One aspect of the present invention includes providing a breakaway support post having one or more slots formed in the support post to allow the support post to buckle or yield in response to forces applied to the support post in a first direction by an impacting vehicle without causing excessive damage to the vehicle. The orientation and location of the slots are selected to allow the support post to effectively anchor the guardrail to direct an impacting vehicle back onto an adjacent roadway in response to forces applied to the support post in a second direction during a downstream rail face impact. For some applications, one or more plates may be attached to the breakaway support post and inserted into the soil to provide additional support during a rail face impact with the associated guardrail and to provide more reliable buckling or yielding of the breakaway support post during a head-on impact with one end of the associated guardrail. Alternatively, the length of the portion of the breakaway support post inserted into the soil may be increased to enhance these same characteristics. For. some applications, the breakaway support post may have a typical I-beam cross section with slots formed in one or more flange portions of the I-beam. Alternatively, the breakaway support post may have a hollow, rectangular or square cross section with slots formed in one or more sides of the post in accordance with teachings of the present invention.
Another aspect of the present invention includes providing a breakaway support post having a first portion or an upper section and a second portion or a lower section with the first portion rotatably coupled with the second portion. A pivot pin or other suitable type of rotatable coupling preferably connects adjacent ends of the first portion and the second portion to allow rotation of the first portion relative to the second portion. The pivot pin is preferably oriented during installation of the associate breakaway support post to allow rotation of the first portion when force is applied thereto in one direction and to block rotation of the first portion when force is applied thereto in a second direction. A shear pin or other suitable releasing mechanism may be provided to releasably couple the first portion and the second portion aligned longitudinally with each other. The shear pin and pivot pin are preferably oriented such that during a head-on impact with the end of the associated guardrail facing oncoming traffic, the shear pin will fail and allow the upper section to rotate relative to the lower section and thus minimize damage to the impacting vehicle. For some applications, a release bar or push bar may be attached to the lower section to assist with disengagement of the upper section from the lower section during such rotation of the upper section. During a rail face impact with the associated guardrail, the same orientation of the shear pin and the pivot pin prevents the upper section from rotating relative to the lower section. Thus, the breakaway support post will buckle or yield during a head-on impact to minimize damage to an impacting vehicle and will remain intact to redirect an impacting vehicle back onto the associated roadway after a rail face impact.
Technical advantages of the present invention include providing breakaway support posts which are easier to initially install and to repair as compared to wooden breakaway support posts. Major portions of each breakaway support post may be fabricated from standard, commercially available steel I-beams using conventional metal bending and stamping techniques in accordance with teachings of the present invention. One or more metal soil plates may be attached to each breakaway support post to further enhance desired characteristics of yielding or buckling during head-on impact with one end of an associated guardrail to minimize damage to an impacting vehicle and to securely anchor the associated guardrail to redirect an impacting vehicle back onto the adjacent roadway after a rail face impact. Breakaway support posts incorporating teachings of the present invention may be used with a wide variety of guardrail end treatments having various types of energy absorbing assemblies located at or near the end of the associated guardrail facing oncoming traffic. For many applications, breakaway support posts may be satisfactorily installed adjacent to the edge of a roadway without the use of steel foundation tubes and/or concrete foundations typically associated with installing wooden breakaway support posts and other types of breakaway support posts.
A further aspect of the present invention includes providing guardrail support posts having a first portion or upper section attached or coupled, at least in part, by a frangible connection, to a second portion or lower section. The support post and frangible connection may be oriented in accordance with teachings of the present invention to resist impact by a motor vehicle from one direction (strong direction), and to yield to impact by a motor vehicle from another direction (weak direction). Preferably, the frangible connection allows the upper portion of the post to deflect slightly and then break off of the lower portion, thus minimizing lifting of the impacting vehicle into the air.